What to do:

Go to mission select and select the mission Breaking Bagram. Play through until you're told that it is the end of the line and you get off the turret in the back of the truck. Once off change weapons by pressing Triangle and use your secondary which is a one shot short range weapon which is perfect for getting this trophy with. Get behind cover like shown in the video and slowly pick off your enemies by aiming onto the head and be PATIENT. Do NOT go too slow though or your AI will take your kills and you will go further into the mission. But if you do fail just press Start and restart from the check point.